1. Field Of The Invention
The invention pertains to an apparatus for control of the output of a cardiomyostimulator and the method for using it.
2. Description Of The Prior Art
A surgical procedure known as Muscle Flap Heart Function Augmentation was pioneered by George Magovern, M. D. This procedure is also known as a cardiomyoplasty and is disclosed and claimed in U.S. Pat. No. 4,791,911 issued Dec. 20,1988 to George Magovern.
In this surgical procedure, the latissimus dorsi is partially dissected from its location in the back while leaving the neurovascular bundle intact. The blood supply and nerve tissues are not severed and remain intact to nourish and excite the muscle. The dissected muscle (also known as and hereinafter referred to as a "muscle flap") is translocated and is passed into the thorax through a rib resection in the axilla. After appropriate midsternotomy, cardiopulmonary bypass etc., the muscle flap is used as a graft muscle in the reconstruction of the cardiac muscle.
For several days postoperatively the muscle flap is permitted to heal before contractions (which might otherwise tear the sutures) are initiated. Subsequently, a protocol of suitable pacing of the muscle flap with a pulsed stimulus is initiated which enables the muscle flap to contribute to the overall cardiac function.
It has been found that the muscle flap can be made to beat in rhythm with the heart in response to signals generated by a type of implantable pulse generator (IPG), as hereinafter described and generically known as a cardiomyostimulator, although it may be known by other names or terms as well. Further, as used in this application, the term cardiomyostimulator is defined to include one or more devices, elements or any combination or configuration that is implanted for the purpose of stimulating graft tissue to augment the cardiac or circulatory function, including but not limited to co-pulsating and counter-pulsating devices. One example of such a device is Model SP1005, marketed under the trademark Cardio-Myostimulator.TM. available from Medtronic.RTM., Inc., of Minneapolis, Minn.
The cardiomyostimulator is both a "receiving" unit and a "sending" unit. As the heart muscle begins its contraction, a sensor which is associated with the heart, senses occurrence of electrical impulses generated by the heart muscle (specifically the depolarization of the atria or ventricle), which immediately precedes and initiates systolic contraction and the sensor in turn generates a signal which is "received" by the cardiomyostimulator. The cardiomyostimulator in turn transfers that signal to the sending unit portion. The muscular channel, or sending unit portion of the cardiomyostimulator, generates one or more pulses either individually or in a burst pattern, and through electrodes implanted in the muscle flap causes the muscle flap to contract in synchrony with the left ventricle to which it is attached, assisting the left ventricle and increasing its output of blood to the body. The cardiomyostimulator is usually implanted in the upper left portion of the abdominal wall.
It has been found by those skilled in the art that over a period of time, the endurance of the muscle flap can be increased thereby causing the muscle tissue of the back to take on more of the characteristics of heart muscle.
Further, it has been found by those skilled in the art (at Allegheny General Hospital of Pittsburgh, Pa. and the Allegheny Singer Research Institute of Pittsburgh, Pa.) that the muscle flap needs to be conditioned so that it can function as heart muscle with contractions stimulated by a signal generated by the cardiomyostimulator, otherwise the muscle flap suffers from fatigue.
Although the process of muscle fatigue is variously defined by those skilled in the medical profession, one definition applicable to the present invention is that fatigue is the process by which a muscle uses energy (while working) at a rate faster than it is replaced (when the muscle rests). For example, typically non-cardiac muscle can work/rest in a ratio of approximately 30%/70% without fatigue. Cardiac muscle by contrast has a work/rest (rest being defined as the period between contraction) ratio estimated at 70%/30%.
Thus it has been found that the muscle flap must be conditioned in order to increase its work ratio to a maximum while minimizing its rest ratio. In light of the foregoing, it was found by those skilled in the art as referenced above, that initiating pulsed stimulus of the muscle flap too soon after surgery (with a stimulus causing strong contraction of the muscle flap) without periods of rest, causes damage to the muscle flap from fatigue by depletion of muscle energy that is not replenished, and ultimately atrophy of the overstimulated muscle flap tissue.
Two schools of thought and two corresponding methods have been developed to overcome this problem of fatigue. One school of thought can be briefly categorized as using a "full strength/ patterned episodes of pulsed stimulus and rest" method. The second school of thought can be briefly categorized as using an "increasing strength/constantly pulsed stimulus" method.
With the first method, for a predetermined healing period after the cardiomyoplasty, the muscle flap is permitted complete rest with no stimulation. At the end of this initial rest period, the muscle flap is subjected to a "training period". The training period is defined as a period of time during which the muscle flap is gradually conditioned for endurance so that it can beat in rhythm with the heart. This training period involves a training schedule which is quite similar to the training schedule an athlete such as a marathon runner would follow. The training schedule consists of alternating episodes of rest and exercise. During the exercise episodes, the muscle flap is stimulated at a sufficient level so as to cause contraction in synchrony with the patient's heart. The exercise episodes continue in ever-increasing duration until the muscle flap has been conditioned and toned to withstand the strain of continuous pulsed stimulation. It is believed by those skilled in the art that if the muscle flap is not continuously paced during the training period, that the skeletal muscle fibers will transform and develop the capacity for contracting in synchrony with the original heart muscle more effectively, for longer periods of time and with less risk of skeletal muscle atrophy.
The second method involves training the muscle flap to contract in synchrony with the heart without any periods of rest. This method begins by supplying a very low level of pulsed stimulus in synchrony with the beating of the patient's heart early during the muscle flap training period. The pulsed stimulus is continuously supplied and progressively increased in intensity until the muscle flap has been conditioned to accept the level of pulsed stimulus which will enable it to contract in synchrony with the patient's heart without fatiguing, i.e. the muscle flap is fully conditioned.
Both of these methods suffer from the limitation that under either method of conditioning and training of the muscle flap, it is necessary for the patient to remain in a specialized health care facility for management of stimulation problems and/or return at extremely frequent intervals in order to effect changes in the duration or intensity of the stimulating signals. This results in severe inconvenience and increased health care costs to the patient in the form of longer hospital stays and inability to lead a normal life-style due to the need to return to the hospital on a daily or nearly daily basis during the "training period".
Regardless of the training method followed, the cardiomyoplasty procedure leads to a need for temporary interruption of the pulsed stimulus for two distinct purposes:
1. Patients often develop a fear or apprehension of an implanted device that causes contraction of tissue which is beyond the patient's control. The patient needs to have a mechanism for controlling the output of the cardiomyostimulator.
2. When the patient is seen by another physician for emergency or other reasons, and that physician does not have access to a device capable of controlling the cardiomyostimulator's output, the physician is helpless to temporarily interrupt the output of the cardiomyostimulator in order to test questionable functioning of the stimulator, questionable response from the muscle flap, or for evaluation of the patient's own heart or heart function (such as by electro-and echo-cardiograms) in the absence of the contractions of the translocated muscle.
Thus, there has arisen in the art the need for a simple, reliable and effective method and device for controlling the output of a cardiomyostimulator which is available to or for patients at all times, and which, while under the direction and supervision of a physician, can be used to control the stimulating device instantly from any location anywhere in the world.